The most important concern is to make sure that the correct concentration of formalin is used. 100% formalin is approximately 37% formaldehyde with methanol added as a stabilizer. The lowest possible percentage of formalin is recommended, often <= 4%.
Absorbed formalin is metabolized to formic acid, which can be toxic. Additives such as methanol may also be toxic if absorbed.
Complications:
(1) acute tubular necrosis, which can be fatal
(2) metabolic acidosis (due to the formic acid)
(3) acute colitis
(4) pain
Risk factors for complications:
(1) high concentration of formalin
(2) high blood levels of formic acid
(3) high volume of formalin solution with prolonged instillation time
Protocol of Sharma et al:
(1) 50 mL aliquots of 4% formalin
(2) instilled in rectum for 3 minutes then removed
(3) 3 instillations (for total mucosal exposure time of 10 minutes)
Protocol of Myers et al:
(1) 30 mL aliquots of 4% formalin
(2) instilled in rectum for 1 minutes then removed
(3) multiple instillations up to 400 mL (13 cycles)
Pikarsky et al used saline irrigation to aid in removal of formalin aliquots.
Some patients may require 2 or 3 treatment sessions for complete control.
